CD97 is a G-Protein Coupled Receptor which is mainly expressed on hematopoietic lineage cells and is upregulated as an early response genein activated T and B cells. The extracellular region of CD97 contains 3-5 EGF-like domains and an RGD motif, suggesting a role for CD97 incell-cell or cell-matrix interactions, although a specific ligand for CD97 which would fufill such a role has not been identified. We are focusing our efforts on establishing 1)the biological role that CD97 plays in immune function, and 2) defining signaling pathways activated by ligation of CD97. In order to address function, we have produced a CD97 null strain of mouse. Early investigations have shown normal development and fertility for these mice. Ongoing studies address the response of CD97 null mice to various forms of immune challenge. Another important aspect of establishing CD97 function is determing its ligand-binding specificity. Using a purified, soluble form of the CD97 extracellular domain, we have shown adhesive activity and motility-inducing activity for melanoma cells. Chemotactic activity and adhesive activity is respectively fully and partially dependent upon the presence of an RGD sequence in CD97 alpha. Current studies are directed at using FITC-tagged CD97 extracellular domain protein as a probe to clone the ligand using an expression-based approach. Important points in defining signaling pathways mediated by CD97 include defining which downstream effectors, including which G-alpha subunits, are activated by CD97 ligation. Treating cells with monoclonal antibody directed against CD97 resulted in stimulation of Tec family tyrosine kinases, most likely through activation of Gq-alpha. We are currently using a dominant negative inhibition approach which relies upon over- expression of G-alpha subnuit cytoplasmic tails to determine which G- alpha subunits are involved in signaling. In addition, anti-CD97 treatment of primary T cells synergistically stimulates zeta chain phosphorylation, IL-2 production, and proliferation. Thus, CD97 may be an antigen co-receptor which is coupled to trimeric G protein-mediated pathways. - immune response, inflammation, G-protein-coupled receptor, null mice,